1. Field of the Invention
The present invention relates generally to the field of radiation detection and imaging technology.
2. Description of Related Art
When a radiation particle (gamma ray, neutron, electron, etc.) is detected in a scintillation detector, the scintillation detector will emit light, which is then converted into an electronic signal by a photosensor (e.g., photomultiplier tube or photodiode). This electronic signal can then be received and processed by electronic circuits. In the period after radiation hits the scintillation detector, the scintillation light decays exponentially with a time constant .tau. (the time when the light level decays to 37% of the onset level), as shown in FIG. 1.
FIG. 1 shows energy output by two gamma ray particles over time. Since the total amount of light emitted by the scintillation detector represents linearly the energy deposited by the radiation particle in the detector, the area or integral under the curves in FIG. 1 is a measure of the particle energy. As shown in FIG. 1, area 5 and area 10 define a measure of the particle energy of the gamma ray particles. Furthermore, the initial peak in the light level is also proportional to the radiation energy. Hence both the area 5 and peak V1 in FIG. 1 may be used to measure the energy of the gamma ray or radiation particle. Since the area under the curve (integral of light) includes many more light signals than the instantaneous peak light level, the integral (the total amount of light emitted) is generally used to measure the radiation energy.
As the radiation flux increases, it becomes increasingly likely that the next radiation particle may arrive at the detector while all previous events are still emitting light (FIG. 2). In this case, the identity of each individual radiation particle will be lost, and several particles will merge into one large signal, as shown in FIG. 2. In this case neither the peak level (V1 or V2 of FIG. 1), nor the integral information (area 5 or area 10 of FIG. 1) can be used to separate or measure the energy of each particle. In these situations, the detection system will fail to respond properly because of erroneous measurement.
It is known that it takes a time period of approximately 4.tau. to collect 98% of the scintillation light from each radiation excitation. Thus, if the next event arrives at time t&gt;4.tau., the pile-up-energy error on the next event will be less than 2%. Hence, to keep pile-up error small, it is desirable to minimize the chance that two events (radiation excitations) will occur in a time period less than 4.tau.. Since the time-lapse between two events is a random distribution (i.e., the time-lapse between two events is a random variable) centered about the "average arrival time", it is generally practiced in the prior art to operate the detector so that the "average arrival time" is 10.times.(4.tau.)=40.tau., to lower the random chance of having two events coming closer than 4.tau.. With this 10.times."head-room", the probability that two events will come closer than 4.tau. would be approximately 10% (using Poisson statistics). The head-room factor as a function of pile-up percentage is shown in Table 1 below:
TABLE 1 HEAD-ROOM FACTOR AND PILE-UPS Head-Room Factor 5 times 10 times 15 times 20 times pile-up/total (%) 18% 10% 6.5% 5%
Thus, a 10.times.head-room is a reasonable choice, and is generally practiced in the prior art. When coupled with a 4.tau. light-collection time (system dead-time), such a prior art detector provides a measured-energy error (due to pile-up) of less than approximately 2% for approximately 90% of the time, and an energy measurement error (energy resolution) greater than 2% for approximately 10% of the time. This minimum 10.times.head-room (40.tau.) timing requirement means that the maximum detection-rate should be less than 1/(40.tau.) for the scintillation detector.
The present invention permits a scintillation detector system to operate at a much higher event-rate (count-rate) by obviating the 10.times.head-room factor without pile-up. The present invention maintains a greater event-rate with little sacrifice in the total amount of scintillation light collected, specifically at a 10 times higher radiation flux with little or no sacrifice in measurement accuracy. If the fraction of scintillation light collected can be reduced (i.e., if a user is willing to compromise measurement accuracy), the present invention allows the detector to count at count-rates approximately twenty times greater than conventional methods.